how can I separate these 2 fingers?
They are not united. They are overlapping.
Well, easy ... not really, no. First you need to select the finger you want to edit. I did from the inside, which I think, is much easier.
Then you can rotate, move, scale the selected nodes/faces. Proportional editing might come in handy. Also, using the 3D-Cursor at the nuckle might help with rotations.
You could also select the middle finger fist, then hide those faces [h] to work on the ring finger - or vice versa. Unhide all faces (in edit mode) [ALT]+[h].
Hint: when you hide faces, switching to sculpt mode will keep those faces hidden.
Also, while you're in edit mode: create a vertex group of each finger. This makes further selections and rigging easier.

Viewed from the inside. Left, selected ring finger. Right, not selected middle finger. They are overlapping at the &quot;first visible&quot; bone.